Make content of AutofillCardUnmaskPrompt scrollable

Per Android doc of ScrollView, it is discouraged to add a ListView
inside a ScrollView. So crrev.com/c/4108590 is reverted.

Add a ScrollView inside autofill_card_unmask_prompt to make it
scrollable.

Demo: https://drive.google.com/open?id=1QfLkZe_HpRQtkdDYFc1UolO94DGPgx3w&authuser=0&resourcekey=0-9qvL_Gbfmr-6Evj-y0iXzA&usp=drive_link

Bug: 1364816, 1402472
Change-Id: I457e085028e685e92837e78dff938d76f798bcff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4117446
Reviewed-by: Theresa Sullivan <twellington@chromium.org>
Reviewed-by: Elizabeth Popova <lizapopova@google.com>
Commit-Queue: Lijin Shen <lazzzis@google.com>
Cr-Commit-Position: refs/heads/main@{#1085992}